Beverage containers and the like are commonly made by blow molding a parison, or preform, that is made from polyethylene teraphthalate (PET) material. Such PET reheat and blow preforms are commonly manufactured by injection molding equipment with either a reciprocating screw or with a two stage injection unit.
The reciprocating screw type is simpler in concept and less expensive to build, and hence is more widely used. In this system, a supply of particulate thermoplastic polymer is maintained in a hopper which feeds the inlet end of a plasticizing screw encased in a heated barrel of constant diameter. Helical flights on the screw convey the particles along the screw where the mass of particles are compressed and rub against the heated barrel surface. This action provides frictional heating as well as some heat conducted from the barrel. However, the bulk of the energy imparted by the screw to the particles is the result of shear. The particles are sheared between two surfaces, the root diameter of the screw and the inner diameter of barrel, which move with respect to each other. The heat generated by this shearing action increases along the length of the barrel as the screw root increases in diameter closer to the outlet end of the screw, so that a homogeneous melted mass of the polymer material is produced.
When the desired amount of polymer accumulates in the space beyond the discharge end of the screw, the screw stops its rotation. It then serves as a plunger, moving forward and forcing a desired quantity of the polymer melt, or a "shot", into a mold or molds with one or more cavities. The polymer melt cools in these cavities and the finished molded articles are ejected from the mold or molds.
The two stage or pre-plasticizing injection system is similar to the reciprocating screw system. However, in this two stage injection system, the role of the plasticizing or extruder screw is restricted to melting the material, with the injection of the molten plastic into the injection mold accomplished by a separate shooting pot and injection plunger or piston system.
The use of a two stage injection system was pioneered by Rees in U.S. Pat. No. 3,117,348. Brown et al., in U.S. Pat. No. 4,966,545, describes a method and apparatus for using a single shooting pot to sequentially deliver two metered injections of the same resin in the same mold cavity. U.S. Pat. No. 4,460,324 to Van Appledorn, describes a method of controlling the injection speed of the plunger in the shooting pot, thereby controlling the injection rate of plastic material into the mold.
For multi-layered applications, U.S. Pat. No. 4,717,324 by Schad, assigned to the assignee of the present invention, describes the application of multiple shooting pots, one for each different resin feeding into each cavity of a multi-cavity mold.
The separation of functions in a two stage injection machine allows an additional degree of freedom to optimize each of the processes in a relatively independent manner. High speed transfer of the melt usually exposes the melt to high shear rates due to resistance to the flow of the melt. Excessive shear rates, and their related heating effect on the melt contribute to the formation of degradation products. The predominant degradation product generated during standard processing of PET is acetaldehyde. Its presence in molded objects such as beverage bottles, water bottles and food containers and the like is very deleterious from a taste standpoint, even at very low concentration levels. Consequently, the major focus of optimization in the plasticizing screw is the reduction of polymer degradation while maintaining melt output.
Separating the plasticization function from the injection function allows the plasticization screw to run in a nearly continuous manner, with the screw only stopping momentarily just prior to melt transfer from the plasticization unit to the injection unit. This mode of operation allows reduced rotational speeds to be employed, as disclosed by Banks in U.S. Pat. No. 5,281,384, and therefore reduces degradation of the molten polymer by reducing the amount of shear heat generated during plasticization. Also, because the screw is operating in a nearly continuous mode, the amount of degradation that occurs is also reduced because the diameter of the screw can be reduced when compared to a standard reciprocating screw injection unit. This size reduction shortens the total melt residence time, or the length of time the material is in a molten state, and hence reduces the degradation products which result from undue heating.
In general, conventional two stage injection systems produce preforms and other articles with acceptable levels of acetaldehyde or other degradation products. Other factors, such as the operating temperature, screw speed, back pressure, melt transfer pressure and melt transfer speed, as well as the particular resin material and the size of the shot can all be important factors in determining the final level of acetaldehyde in the melt When all of these factors are combined, the prior art methods were found to meet specifications, but always at the upper end of the acceptable range of acetaldehyde or other degradation products, making the process much harder to control. Thus, a need exists for a solution to reduce the acetaldehyde and/or other degradation content of the finished product.